class_docstr: A size of a financial instrument for use in trading.
  
  Parameters
  ----------
  instrument : `Instrument`
      The unit of the quantity.
  size : `Decimal`
      The number of units of the instrument.
  path_id : str, optional
      The path order_id that this quantity is allocated for and associated
      with.
  
  Raises
  ------
  InvalidNegativeQuantity
      Raised if the `size` of the quantity being created is negative.
code: def __sub__(self, other: "Union[Quantity, Number]") -> "Quantity":
      return Quantity._math_op(self, other, operator.sub)
  
imports: import operator
  from typing import Union, Tuple, Callable, TypeVar
  from numbers import Number
  from decimal import Decimal, ROUND_DOWN
  from functools import total_ordering
  from tensortrade.core.exceptions import InvalidNegativeQuantity, IncompatibleInstrumentOperation, InvalidNonNumericQuantity, QuantityOpPathMismatch
label_desc: Use image node_id 13 for calling the Quantity obj's underlying member method code with example usage: obj.__sub__(other) and returns: Quantity